diff --git a/gallery_dl/postprocessor/metadata.py b/gallery_dl/postprocessor/metadata.py
index 5004bed6..93dd9a19 100644
--- a/gallery_dl/postprocessor/metadata.py
+++ b/gallery_dl/postprocessor/metadata.py
@@ -206,7 +206,8 @@ class MetadataPP(PostProcessor):
             sort_keys=options.get("sort", False),
             separators=options.get("separators"),
             indent=options.get("indent", indent),
-            check_circular=False, default=str,
+            check_circular=False,
+            default=util.json_default,
         )
 
 
diff --git a/gallery_dl/util.py b/gallery_dl/util.py
index 62e7b4aa..6255d49e 100644
--- a/gallery_dl/util.py
+++ b/gallery_dl/util.py
@@ -223,8 +223,14 @@ def datetime_to_timestamp_string(dt):
         return ""
 
 
+def json_default(obj):
+    if isinstance(obj, CustomNone):
+        return None
+    return str(obj)
+
+
 json_loads = json._default_decoder.decode
-json_dumps = json.JSONEncoder(default=str).encode
+json_dumps = json.JSONEncoder(default=json_default).encode
 
 
 def dump_json(obj, fp=sys.stdout, ensure_ascii=True, indent=4):
@@ -233,7 +239,7 @@ def dump_json(obj, fp=sys.stdout, ensure_ascii=True, indent=4):
         obj, fp,
         ensure_ascii=ensure_ascii,
         indent=indent,
-        default=str,
+        default=json_default,
         sort_keys=True,
     )
     fp.write("\n")
diff --git a/test/test_util.py b/test/test_util.py
index 0813a0bc..780f4751 100644
--- a/test/test_util.py
+++ b/test/test_util.py
@@ -750,6 +750,7 @@ def hash(value):
         self.assertIs(obj(), obj)
         self.assertIs(obj(1, "a"), obj)
         self.assertIs(obj(foo="bar"), obj)
+        self.assertEqual(util.json_dumps(obj), "null")
 
         i = 0
         for _ in obj: